1. Field of the Invention
This invention relates to a terminal structure of a flexible printed circuit board of the type having electrically conductive patterns formed on a thermoplastic insulative film.
2. Prior Art
Much progress has been made recently in reducing the size and thickness of electronic components used in electronic devices. Flexible printed circuit boards for mounting these components are widely employed since they are very useful in terms of utilizing space effectively. Specifically, flexible printed circuit boards are widely used in the construction of membrane switches, flat cables and the like.
A flexible printed circuit board of the aforementioned kind basically comprises a flexible insulative film and electrically conductive patterns formed on the film. Flexible printed circuit boards in general use include the following two types:
(1) a flexible printed circuit board comprising a substrate obtained by affixing a copper foil to a polyimide film, and electrically conductive patterns formed on the substrate by an etching process and PA1 (2) a flexible printed circuit board comprising a polyester insulative film and electrically conductive patterns formed on the film by the screen printing therein of an electrically conductive paste such as silver paste.
The flexible printed circuit board of type (1) is disadvantageous in that a high cost is involved in the manufacturing process, which entails affixing the copper foil to the polyimide film having a high heat resistance and etching away the film to form the conductive patterns. To reduce the cost, the general practice is to employ the cheaper polyester film used in the flexible printed circuit board of type (2).
Since the polyimide film used in the flexible printed circuit board of type (1) is heat resistant, metallic terminal members which will form the terminals of the board are generally are soldered onto the conductive patterns at a terminal section. However, polyester film has comparatively little resistance to heat, so the metallic terminal members cannot be directly soldered onto the conductive patterns at the terminal sections. For this reason, an arrangement of the type shown in FIG. 9 is ordinarily is employed. Here, the terminal section of a flexible printed circuit board 72 is clamped by a connector 71 having terminals 73, and contacts connected to the terminals 73 inside the connector 71 are brought into pressured contact with the conductive patterns at the terminal section. The terminals 73 thus serve as the terminals of the flexible printed circuit board 72. An alternative method is to mechanically press the terminal section of the flexible printed circuit board into pressured contact with the electrodes of an electronic component to effect a direct connection with the terminal section.
The arrangement shown in FIG. 9 has certain drawbacks. Specifically, the connector 71 for clamping the terminal section of the flexible printed circuit board 72 occupies a large amount of space and therefore is an obstacle to a further reduction in size and thickness. The connector 71 also is costly.
Accordingly, if the metallic terminal members are electrically or mechanically fixed to the terminal section of the flexible printed circuit board of type (2) using the polyester film, the connector may be dispensed with, solder can be applied to the metallic terminal members and the arrangement will occupy less space than that using the connector 71. Though several methods of securing the metallic terminal members are available, all of them have certain shortcomings.
Specifically, one method is to connect the metallic terminal section by-using an electrically conductive adhesive of, for example, the hot-melt type. However, such a connection has little mechanical strength and must be reinforced. Though the connection can be reinforced by hardening it with an insulative bonding agent, this approach is laborious and costly.
The method of mechanically bringing the terminal section of the flexible printed circuit board into mechanical pressured contact with the electrode portions of electronic components involves difficulties in terms of assuring a reliable connection between the contacting surfaces of the conductive patterns at the terminal section and the electrode portions of the electronic components. Also the connection between the contacting surfaces is adversely affected by flux build-up when solder is applied to the electrode portions.